A Universal Mobile Telecommunications System (UMTS) or third generation (3G) network can be separated into a number of major components, namely one or more core networks which are responsible for setting up and controlling user sessions, and a UMTS radio access network (UTRAN) which controls access to the air interface. The interface between UTRAN and user equipment (UE) is provided by nodes that may be referred to as “Node B” (analogous to base stations in 2G/GSM networks) or base stations. NodeBs are responsible for transmitting and receiving data over the air interface and are controlled by radio network controllers. User and control data is routed between a base station and a core network via the base station and the radio network controllers. The interface between a base station and a radio network controller is referred to as the Iub interface. The interface between two radio network controllers in the same network is referred to as the Iur interface. A Iu interface carries user traffic (such as voice or data) as well as control information, and is mainly needed for soft handovers. Soft handover refers to a feature used by the Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA) standards, where a user equipment, such as a cell phone, is simultaneously connected to two or more cells (or cell sectors) during a call. On the uplink (user equipment-to-cell-site), all the cell site sectors that are actively supporting a call in soft handover send the bit stream that they receive back to the radio network controller, along with information about the quality of the received bits. The radio network controller examines the quality of all these bit streams and dynamically chooses the bit stream with the highest quality. Again, if the signal degrades rapidly, the chance is still good that a strong signal will be available at one of the other cell sectors that are supporting the call in soft handover.
In UTRAN, the high-speed downlink shared channel (HS-DSCH) does not use soft handover as dedicated channels do. Instead, a procedure called HS-DSCH serving cell change (HSCC) is utilized to make a hard handover. Soft handover is still used for the uplink, and an active set is managed in the same way as for non-high speed user equipment. The active set comprises all cells the user equipment is connected to in uplink soft handover. The user equipment continuously measures the common pilot (CPICH) and comprises a hysteresis to be fulfilled during a certain time (time to trigger). The active set update procedure is schematically illustrated in FIG. 1. The active set update procedure is triggered by measurement report 1a, 1b or 1c, 101 informing the radio network controller 315 that new cells have fulfilled the criterions to be added (measurement report 1a), deleted (measurement report 1b) or replaced (measurement report 1c) in the active set. The measurement report 1a, 1 b, 1c is sent from the user equipment 310 to a network controller 315, e.g. a serving radio network controller SRNC. A serving radio network controller 315 is a type of radio network controller serving particular user equipment 310 and manages the connections towards that user equipment 310. When in HS-DSCH operation, the downlink is not in soft handover. Instead, one of the cells (typically the strongest) in the active set is marked as current HS-DSCH serving cell. The network controller 315 then performs a radio link addition 102, and sets up the required radio links by sending and receiving setup request and response to/from the base stations 350, 307. The network controller 315 transmits the active set update message 105 to the user equipment 310. When the user equipment 310 has received the active set update from the network controller 315, it prepares 106 a processing, i.e. it reads the message and applies the new configuration, e.g. adds or deletes a radio link. The user equipment 310 sends an active set update complete message 107 to the network controller 315 confirming that the active set update was complete. The duration of an active set update procedure may, as an example, be calculated as follows:Tasu=Ttrig1a+2*TUu+2*TIub where                Ttrig1a=Time to trigger measurement report 1a=320 ms        TuUu=Uu (Radio Interface) delay=100 ms        TIub=Iub (Radio Network Controller—NodeB Interface) delay=10 ms        Tasu=Time for active set update        
The signalling sequence for a (regular) HSCC procedure for hard handover is schematically shown in a combined flow and signalling diagram in FIG. 2. The user equipment 310 performs a handover evaluation 200 to determine whether a handover shall be performed. This is triggered by a neighbour cell (target cell) being stronger than of the current cell (serving cell). A measurement report 1d is then sent 201 from the user equipment 310 to the network controller 315, e.g. the serving radio network controller (SRNC), indicating that another cell in the active set has become the strongest one. As an example, the measurement report 1d is triggered to be sent from the user equipment when the measured common pilot channel level (CPICH) of the target cell is stronger than the current cell by a certain hysteresis for a given time, governed by a parameter Ttrig1d (time to trigger measurement report 1d).
When not considering possible processing delays in the network controller 315 and the user equipment 310 (i.e. in simulation), the delay of the cell change procedure, Tcc, used may, as an example, be calculated as follows:Tcc=Ttrig1d+TUu+2*TIub+TActivationTime where                Tcc=Delay of cell change procedure        Ttrig1d=time to trigger measurement report 1d         TUu=Uu (Radio Interface) delay=100 ms        TIub=Iub (Radio Network Controller—NodeB Interface) delay=10 ms        TActivationTime=Activation time        
When the network controller 315 receives the measurement report 1d indicating the existence of this stronger cell, the network can take the decision to change the serving cell, i.e. it takes a handover decision 202. When a cell change is triggered, the network controller configures source and target base stations (shown as only one base station in FIG. 2) with the new configuration and configures the Iub transport bearer. The network controller 315 sends a radio link reconfiguration prepare 203 message to the base stations 305, 307, and receives in return a radio link reconfiguration ready message 204. When both base stations 305, 307 (serving and target) have acknowledged the configuration, the network controller 315 calculates 205 the activation time for the new configuration in case the switch to the new configuration is a synchronized procedure, meaning that the user equipment 310 and the network controller 315 i.e. serving network controller, shall move to the new configuration at the same time. The activation time is relative to a connection frame number (CFN). An offset is needed to cover for the time it takes to transmit the re-configuration messages 206 to both the user equipment 310 and the base stations 350, 307. The network controller 315 sends a physical channel reconfiguration message 207 to the user equipment 310. The user equipment 310 performs a processing 206, i.e. it reads the message from the network controller 315 and applies the new configuration, e.g. adds or deletes a radio link 208 for the handover and sends a physical channel reconfiguration message 209 to the network controller 315.
There is however a problem for user equipments 310 travelling at very high speed, since the link quality of the source cell (i.e. serving cell) may degrade before the cell change procedure to the target cell is completed. If this happens before the network controller 315 is able to successfully transmit the Physical Channel Reconfiguration message 209, the network controller 315 will no longer be able to reach the user equipment 310 and the call will be dropped.
Enhancements to the HS-DSCH serving cell change procedure are consequently required regarding radio protocol procedures and structures, Iub/Iur protocols and User equipment, base station and radio resource management (RRM) performance requirements.
Several improvements exist to the high speed serving cell change procedure to reduce drop rates, some of which are shortly listed below:                Transmit of the handover command in the target cell.        Proactive retransmissions for delay sensitive packets such as signalling radio bearer (SRB) packets.        Bi-casting handover commands from both the serving Node B and the target Node B        
Any improvements and modifications, including those mentioned above, to the regular HS-DSCH serving cell change procedures as exemplified in FIG. 2 are hereinafter referred to as modified cell change procedures within the context of the description of the present solution. Common for the above improvements is that they come at a certain cost in system performance. Bi-casting reduces capacity on radio and transport network, proactive retransmissions cost radio resources, and the target cell re-pointing requires resource reservation in all cells in the active set.